Membrane separation systems are used for a wide variety of processes in which it is desired to remove or reduce the concentration of certain materials in a fluid. In particular they are used to purify waste water effluents from industrial and domestic water use. They are also increasingly being used for producing fresh water from sea water or brackish water supplies by removing the salt, and other contaminants, from saline feed water.
The performance of membrane separation systems depends on many operational parameters such as the:
pressure of the feed fluid,
pressure of the permeate fluid (or flux),
pressure of the discard (or sludge),
cross flow velocity, the concentration,
the nature of the materials to be removed,
temperature etc.
The performance is also dependent on the intrinsic properties of the membrane material.
The overall operational capacity of a membrane increases with surface area. The size of the modules incorporating the membranes also increases with the area of the membranes. In one of the most common physical configurations the ratio of the membrane area to the volume of the module is minimized by wrapping the membranes in a spiral around a central permeate collection tube. In these so called “Spiral Wound” modules the membrane surfaces are kept apart by a spacer fabric which is usually made of polymer strands.
During operation, the surfaces of the membrane become covered with materials present in the feed solution to which the membrane is highly impermeable. The flux or fluid permeating through the membrane is then largely devoid of such materials. The materials rejected by the membrane are swept along by the cross flow and appear in the discarded fluid (also referred to as the sludge).
Under some circumstances, and after prolonged use of the system, some of the materials that are present in the sludge at high concentrations become attached to the membrane and in some circumstances become permanently attached to the surface. This is known as “Fouling”. Fouling leads to a reduced permeate flux and can also lead to a decrease in the rejection characteristics of the membrane. Any reduction in the rejection characteristics has a significant negative impact, particularly on the performance of so called “Reverse Osmosis” membranes for removing salt from sea water or brackish feed water.
In particular, fouling in such membranes will give rise to reduced salt rejection and hence lower quality (higher salinity) permeate fluxes. Many factors contribute to the fouling process, including dynamic effects arising from concentration polarization layers at the membrane surface. From an operational point of view it is important to monitor the performance of the system and to take remedial steps to remove fouling of the membranes. The studies of Li et al (Journal of Membrane Science. 149 pp. 83-97, 1998), Kwon, Vigneswaran, Fane and Ben Aim R., in Separation and Purification. Technology, 19 pp. 169-181 (2000) and Zhang et al (Journal of Membrane Science. 282; 189-197, 2006) teach that the rate of fouling dramatically increases when a system is operated so that the permeate flux exceeds certain critical values.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.